The present invention relates to chemical delivery systems, and in particular to an apparatus and method for delivering high-purity or ultra-high purity chemicals to a use point, such as a semiconductor fabrication facility or tool(s) for chemical vapor deposition. Although the invention may have other applications, it is particularly applicable in semiconductor fabrication.
Semiconductor manufacturers require chemicals having at least a high-purity for production processes to avoid defects in the fabrication of semiconductor devices. The chemicals used in the fabrication of integrated circuits usually must have an ultra-high purity to allow satisfactory process yields. As integrated circuits have decreased in size, there has been an increase in the need to maintain the purity of source chemicals.
One ultra-high purity chemical used in the fabrication of integrated circuits is tetraethylorthosilicate (TEOS). The chemical formula for TEOS is (C.sub.2 H.sub.5 O).sub.4 Si. TEOS is used widely in integrated circuit manufacturing operations such as chemical vapor deposition (CVD) to form silicon dioxide films.
Integrated circuit fabricators typically require TEOS with 99.999999+%(8-9's+%) purity with respect to trace metals. Overall, the TEOS must have a 99.99+% purity. This high degree of purity is necessary to maintain satisfactory process yields. It also necessitates the use of special equipment to contain and deliver the high-purity or ultra-high purity TEOS to CVD reaction chambers.
High-purity chemicals and ultra-high purity chemicals, such as TEOS, are delivered from a bulk chemical delivery system to a use point, such as a semiconductor fabrication facility or tool(s). A delivery system for high-purity chemicals is disclosed in U.S. Pat. No. 5,465,766 (Seigele, et al.). (Related patents issued to the same inventors and assigned to the same assignee are U.S. Pat. Nos. 5,562,132; 5,590,695; 5,607,002; 5,711,354; and 5,878,793.) The system comprises: a bulk canister located in a remote chemical cabinet with a delivery manifold/purge panel; a refillable stainless steel ampule to supply high-purity source chemicals to an end user; and a control unit to supervise and control the refill operation and to monitor the level of the bulk container. The system has two basic modes of operation: (1) a normal process operation during which high-purity source chemical is supplied to the end user; and (2) the refill mode of operation during which the refillable stainless steel ampule is refilled with high-purity chemical.
The bulk container is continuously pressurized with an inert gas (e.g., helium), which forces the high-purity source chemical from the bulk container through a refill line and to the ampule. A metallic level sensor assembly in the ampule contains a metallic level sensor. The metallic level sensor preferably is a dual level sensor capable of detecting two separate levels of source chemical in the ampule and has two trigger points--a "high level" (full) condition and a "high-high level" condition.
A metallic level sensor assembly for the bulk container comprises a dual level metallic level sensor with trigger points, which provide signals indicating the levels of high-purity chemical in the bulk container. At least one of the trigger points generates a "low level" signal indicative of a level at which the bulk container should be replaced. The sensor is a metallic float level sensor, which includes a metallic float slidably mounted on a metallic shaft. The metallic float rises and falls as the level of high-purity chemical rises above one of the trigger points and drops below one of the trigger points. One of the trigger points is for detecting when the high-purity chemical is near the "empty" level in the bulk container and another trigger point is for detecting when the high-purity chemical is at a "low level" in the bulk container.
It is desirable to determine when the bulk container in such delivery systems is "empty" for several reasons. First, the customer wants to use as much chemical from each container as possible for economical reasons. Second, to avoid any interruption in operations, it is desirable to replace the bulk container as soon as possible after it reaches empty. Also, complete use of the chemical in the bulk container avoids potential problems associated with disposal of chemicals left in the container after it is removed from service.
A weigh scale may be used to determine when the bulk container is approaching "empty." However, the purchase of a weigh scale means additional capital investment. Also, such a method of determining the approach of "empty" usually results in leaving a liquid heel in the container, which is not desirable.
Metallic float sensor assemblies, such as in the patents issued to Seigele, et al., are known sources of metallic particles, which are contaminants in the electronics industry. Sliding metal-to-metal contact causes the shedding of metal particles and dissolution of metal ions, thus contaminating the high-purity TEOS or other high-purity source chemical in the delivery systems. In addition to being generators of contaminants, float level sensors do not operate well in chemicals having relatively high viscosities (e.g., tantalum pentaethoxide, TAETO).
There are various other types of level sensors used for detection of an "empty" or "approaching empty" condition. The different types of sensors include optical, reed/float, capacitance, differential pressure, and load cells/strain gauges. There are disadvantages associated with each of these types of sensors. For example, differential pressure and load cells/strain gauges only detect down to about 3% to 5% level. Optical, reed/float, and capacitance sensors require a probe, a potential source of contaminants, to be inserted in the chemical supply, and these sensors also typically detect only down to about 3% to 5% level. The use of a probe also requires elastomeric seals or metal seals, both of which may leak.
Capacitance level sensors also are subject to interference from outside signals and "noise", such as that from radio frequency induction (RFI) and electromagnetic induction (EMI), both of which are common in semiconductor fabrication facilities.
Another attempt to enable 100% chemical usage from bulk containers has involved installation of a well in the bottom of the container and placement of a dip tube and level sensor in the well. Such containers are more expensive, are harder to clean, require additional height, and still do not enable 100% usage of the chemical.
The use of optical liquid sensors to detect liquid in teflon tubing is well known in the art of chemical delivery systems. For example, in a system for delivering a liquid chemical for cleaning semiconductor wafers, such as sulfuric acid (H.sub.2 SO.sub.4), optical liquid sensors may be used for liquid level detection in a pressure vessel and also for liquid flow detection in teflon tubing throughout the system.
Such a delivery system may include a supply of liquid chemical in a drum connected by a teflon tube to a pump for pumping the liquid chemical from the drum through lines and filters to a fabrication facility or other end use. An optical liquid flow sensor on the teflon tube is commonly used to prevent cavitation and/or dry runs that could damage the pump. The optical sensor activates an electronic switch which shuts the pump off when no liquid is flowing in the line, a condition typically indicated first by a breakup in liquid flow when the drum level approaches empty.
Such systems and detection methods are not suitable for delivering high-purity and ultra-high purity chemicals used in integrated circuit manufacturing operations such as CVD to form silicon dioxide films. These types of applications cannot use the teflon tubing/optical sensor method of detecting liquid fluid because of the sensitivity of the high-purity and ultra-high purity chemicals to atmospheric contamination (e.g., O.sub.2 and H.sub.2 O) that would diffuse into the system. To avoid such contamination, stainless steel systems typically are used.
In addition, delivery systems using in-line pumps generally are unacceptable for high-purity and/or ultra-high purity chemicals because the pumps are a source of contamination and often have maintenance problems. For those and other reasons, chemical delivery systems which operate without in-line pumps have been designed. For example, U.S. Pat. No. 5,148,945 (Geatz) ("the '945 patent") discloses an apparatus and method for the transfer and delivery of high-purity chemicals using a combination of vacuum and pressure transfer from a bulk source through one or more intermediate pressure/vacuum vessels to one or more end-use stations. The use of multiple vessels allows simultaneous delivery of chemical to the end-users and refilling of the vessels.
Each of the vessels is equipped with a level sensor, preferably a capacitive-type level sensor, for monitoring fluid level in the vessel. Means are provided to switch between delivery vessels to maintain a continuous supply to a delivery conduit when a supplying delivery vessel "approaches" empty (rather than reaches empty). However, the '945 patent does not teach a method or means for substantially complete utilization of the chemical in one vessel (i.e., for obtaining an "empty" condition) before switching over to another vessel.
The '945 patent discloses the use of an optional flow control on the delivery conduit to control the flow rate to the end users The flow control includes a flow sensor, which may be an ultrasonic flow sensor. However, the flow sensor is not used to determine fluid level in any of the vessels of the system. Rather, fluid levels in the vessels are determined by level sensors, preferably capacitive-type level sensors.
Two related patents assigned to the same assignee, U.S. Pat. No. 5,330,072 (Ferri Jr., et al.) and U.S. Pat. No. 5,417,346 (Ferri Jr. , et al.), teach improvements of the process and apparatus disclosed in the '945 patent, the improved process and apparatus being electronically controlled. As before, level sensors (preferably capacitive-type sensors) are used to monitor liquid level in the pressure/vacuum vessels. When a vessel "approaches" empty, a signal is provided from a "low" level sensor and another vessel is brought on-line so that flow continues without interruption. The near-empty vessel is then refilled. Flow from the first vessel is shut down to prevent it from "over-emptying".
It is desired to have a more reliable method of detecting an "empty" condition in a bulk reservoir of a chemical delivery system for high-purity or ultra-high purity chemicals.
It is further desired to have a more reliable method of delivering a high-purity or ultra-high purity chemical from a bulk chemical delivery system to a use point.
It is still further desired to have an apparatus and a method which deliver high-purity or ultra-high purity chemicals which minimize chemical waste and disposal costs.
It is still further desired to have a method of delivering high-purity or ultra-high-purity chemicals which has a lower cost for abatement.
It is still further desired to have an apparatus and method for delivering high-purity or ultra-high purity chemicals which overcome the difficulties and disadvantages of the prior art to provide better and more advantageous results.
It is still further desired to have a chemical delivery system for delivering high-purity or ultra-high purity chemicals which does not require the use of level sensors on or within the containers/reservoirs of the system.
It also is desired to have an improved apparatus and method to deliver high-purity or ultra-high purity chemicals to semiconductor manufacturing processes using an operationally safe, cost-effective, bulk chemical delivery system.